The Missile Defense Agency (MDA) has its origins in the Strategic Defense Initiative (SDI) which was established in 1983 by Ronald Reagan which was headed by Lt. General James Alan Abrahamson. Under the Strategic Defense Initiative's Innovative Sciences and Technology Office[2][3][4] headed by physicist and engineer Dr. James Ionson,[5][6][7][8] the investment was predominantly made in basic research at national laboratories, universities, and in industry. These programs have continued to be key sources of funding for top research scientists in the fields of high-energy physics, supercomputing/computation, advanced materials, and many other critical science and engineering disciplines—funding which indirectly supports other research work by top scientists, and which was most politically viable to fund within the Military budget of the United States environment.[9] It was renamed the Ballistic Missile Defense Organization in 1993, and then renamed the Missile Defense Agency in 2002. The current commander is U.S. Air Force Lt. Gen. Samuel A. Greaves.

The Missile Defense Agency's mission is to develop, test and prepare for deployment of a missile defense system. Using complementary interceptors, land-, sea-, air- and space-based sensors, and battle management command and control systems, the planned missile defense system will be able to engage all classes and ranges of ballistic missile threats. Our programmatic strategy is to develop, rigorously test, and continuously evaluate production, deployment and operational alternatives for the ballistic missile defense system. Missile defense systems being developed and tested by MDA are primarily based on hit-to-kill technology. It has been described as hitting a bullet with a bullet – a capability that has been successfully demonstrated in test after test.

Arrow 3, a joint effort between the U.S. and Israel, launched in January 2014

Ballistic Missile Defense Systems (BMDS) must be able to be operated in different regions of the world in order for the success of the MDA mission. The International Strategy was approved by the MDA Director in 2007. The general strategy for international efforts is:[10]

Outreach: Communicate the importance of missile defense by promoting worldwide BMDS by sharing information with allies and partners.

Ballistic missile systems using advanced liquid- or solid- propellant propulsion are becoming more mobile, accurate and capable of striking targets over longer distances and are proliferating worldwide.[12]:pp.18–19/61

Iran currently has short- and medium- range missiles with guidance systems. Iran's launch of a medium range, solid fuel ballistic missile demonstrates its ability to hit targets in Israel and southern Europe.[13] Iran also successfully launched the Safir Space Launch Vehicle on Feb 2, 2009. It was then speculated that development of an ICBM was not far behind. Intelligence reports that a missile could have been built sometime between 2010 and 2015 perhaps using help from Russian and North Korean technology.[14][15]

North Korea currently deploys a Nodong ballistic missile capable of hitting Japan and South Korea, and is developing a new intermediate-range ballistic missile (IRBM) which could reach Guam and the Aleutian Islands. They also successfully demonstrated the staging and separation technologies required to launch a Taepo-Dong 2 ICBM which has the capability to reach the United States.[16] The Taepodong missile was first tested in 2006, and failed 40 seconds into midflight. North Korean missiles are notoriously unreliable, and many of the DPRKs missile tests have failed, including the most recent Taepodong-2 launches in 2009 and 2012,[17] and a failed launch of the BM25 Musudan in 2016.[18] On 1 January 2017, North Korea announced its final preparation for a test of an ICBM, for the first time.[19] On 6 March 2017, North Korea launched four missiles from Tongchang-ri,[20] a known long-range missile site at 7:36 a.m. local time, one of which landed in the Sea of Japan, with the remaining three missiles landing in Japan's economic zone.[21] On July 4, 2017 North Korea launched a ballistic missile that had the potential to be an ICBM. It flew into space and landed in the Sea of Japan. "The launch continues to demonstrate that North Korea poses a threat to the United States and our allies," a Pentagon statement said.[22]

Syria has been identified as a host for Short Range Ballistic Missiles (as it acquires equipment from North Korea and Iran).[12]:p.19/61

MDA divides its systems into four phases, boost, ascent, mid-course and terminal, each corresponding to a different phase of the threat ballistic missile flight regime. Each phase offers different advantages and disadvantages to a missile defense system (see missile defense classified by trajectory phase), and the geography of each defended area dictates the types of systems that can be employed, thus the flexible and layered defense approach concept should improve overall defense effectiveness. The more opportunities you have to shoot it down, the better the chance of success.

Can intercept all ranges of missiles, but the missile boost phase is only from one to five minutes. It is the best time to track the missile because it is bright and hot. The missile defense interceptors and sensors must be within close proximity to the launch, which is not always possible. This is the most desirable interception phase because it destroys the missile early in flight at its most vulnerable point and the debris will typically fall on the launching nations' territory.

Ascent phase

This is the phase after powered flight but before the apogee. It is significantly less challenging than boost phase intercepts, less costly, minimizes the potential impact of debris and reduces the number of interceptors required to defeat a raid of missiles.

Midcourse phase

This phase begins after the booster burns out and begins coasting in space. This can last as long as 20 minutes. Any debris remaining will burn up as it enters the atmosphere. Ground based missile defense systems can defend from long-range and intermediate-range ballistic missiles in this phase. Mobile elements can defend against medium and short ranged missiles in midcourse.

Terminal phase

This phase is the last chance to intercept the warhead. This contains the least-desirable Interception Point (IP) because there is little room for error and the interception will probably occur close to the defended target.[10]

Kinetic Energy Interceptor (KEI) – in December 2003, MDA awarded a contract to Northrop Grumman for developing and testing. It will have to be launched from a location not too far from the launch site of the target missile (and is therefore less suitable against large countries), it has to be fired very soon after launch of the target, and it has to be very fast itself (6 km/s). In 2009, the Department of Defense and MDA determined that Northrop Grumman could never build anything this technologically advanced and has cancelled the program, allocating no funding for it in its later budget submission.[26]

Boeing YAL-1 Airborne Laser (ABL) – Team ABL proposed and won the contract for this system in 1996. A high-energy laser mounted on a converted 747 airliner was used to intercept a test target in January 2010,[27] and the following month, successfully destroyed two test missiles.[28] While the program has been cancelled due to concerns about its practicality with present technology (while successful the system was still extremely short ranged, likely needing to fly in heavily defended space to make an interception) the YAL-1 served to demonstrate the potential of such a system. Being able to be deployed very quickly to any part of the world makes a potential future system extremely attractive, as does the ability to intercept a large number of missiles.

Ascent phase intercept (API) – Emerging intercept technologies are being developed and designed to defeat launched missiles in their ascent phase. This phase is after the boost phase and prior to the threat missile’s apogee (midcourse). The Ascent phase intercept program is still classified so there is little information on it.